Flash diffuser caps: so misunderstood!

One of the most popular accessories for a speedlight flash is a white translucent plastic cap that sits over the end. Some flash manufacturers include it when you buy the flash, otherwise you have to buy it yourself (Sto Fen is one well-known brand). I see a lot of photographers using them who clearly have no idea what they do and when they should be used.

When I teach basic flash concepts, I like to hold up one of these caps and ask what it is. Most people will call it a diffuser. When I then ask them what does it do, they often hesitate: “it diffuses the light?” If I then ask what that means I often get a variety of vague guesses that only underlines how they don’t really understand it at all.

Some people will say that it softens the light, but what does that mean? What is soft and hard light exactly?

To answer, consider this question: what’s the biggest light source you’ve ever used? Most of the time photographers will say the sun, which is wrong. I understand why they say it (basic science says the sun is indeed massively huge) but when a photographer shoots outdoors in the bright sun, they’re actually working with a very small (and very bright) dot in the sky. The sun is a very long away so perspective makes it seem quite small to us, just like any distant object appears small.

What do the shadows cast by the sun look like? Most people will say they’re harsh but what does that mean? It simply means that there is a sharp difference between the lit and shadow areas. An object will cast a shadow with a clear shape that has crisp edges. So we can conclude that the sun, a small light source, creates hard light because the shadows have hard edges.So what’s the biggest light source then? I’ll suggest it’s the sky on an overcast day. When you look up, all you see is a massive bank of white clouds glowing. What kind of shadows do we get on an overcast day? When you look at the ground, distinct shadows are harder to see – they’re all vague and fuzzy and they might blend into each other. When shadows have a soft edge, we call that soft light.So… a big light source gives us soft light. A small light source gives us hard light. How can a diffuser cap on a flash make the light softer?The simple answer is: it doesn’t make the light softer. Put the diffuser cap over the end of the flash and the flash doesn’t get much bigger.So what does it do then? Consider these two basic light principles:

A bare speedlight fires light forwards only. No light goes backward because it can’t bend in mid-air (see rule 2).
  1. If you can see the light, it can see you
  2. Light travels in a straight line

If you put that cap over the flash and fire it, where can you see that cap glowing? If the flash is aimed at you, you’ll see the end of the cap glow. If you walk around to the left or right, you’ll see each side of the cap glowing. If you were above the flash, you’d see it glowing. The glowing cap on the end of the flash is visible from all around, therefore it must be throwing light all around (see rule 1 above). The flash is no longer aimed in just one direction, it’s spreading the light to spray out in every direction.

The cap is really a light spreader, not a softener.

So let’s imagine you’re outside in the open in a park. You point the flash (with cap) at your subject. Some of the light comes out of the end and hits your subject. Some of the light sprays out the sides and goes left/right into the open and never comes back. Some of the light sprays out the top into the sky and never comes back. Some of the light sprays down and gets absorbed into your black lens or hits the ground.

Adding a diffuser cap causes the light to spray in every direction

This is utterly pointless.

Standing outside, all you’ve done is spray a lot of light everywhere and it hasn’t ended up in your photo. It’s a waste of light but it’s also a waste of battery power. Why use a flash to generate all that light just to throw most of it away? Yes, some of your light is hitting your subject but it was going to do that without the cap anyway. All you’ve done is made that direct light have to fight through the plastic to reach your subject. Your poor flash is working it’s butt off for no reason.

So why the hell do they sell these caps if they don’t do anything useful?

Spraying the light everywhere is only useful if that light comes back into the photo.

From above: pointing the flash up with a diffuser cap allows the light to spray everywhere where it then bounces off the walls (and ceiling) to make the whole room the light source.

Fire the flash indoors with a diffuser cap and the light will spray up to the ceiling, bounce off that and hit your subject. It will also hit the walls left/right and bounce back to your subject. With all of that light bouncing around the room, you’ve turned the whole room into your light source. The room is bigger than your flash so now you have a softer light. Hurrah!

So, yes, a diffuser cap can give you softer light, but not by itself. It needs all of those surfaces around to bounce the light back into your shot. A photographer needs to look around their environment and assess whether there is any point spraying light everywhere. Will it come back? Or will you just be throwing light away? If you’re just going to throw light away, leave the cap in your bag.

I once had an experienced professional photographer tell me that he used a diffuser cap to soften the light and to reduce the amount of light hitting his subject. As explained above, you need the right conditions to soften the light. His argument about reducing the amount of light from the flash? That doesn’t make sense: if you want less light from the flash, just turn down the flash exposure compensation (FEC) so that the flash generates less light. There’s no point asking the flash to produce lots of light if you’re then intending to throw most of it away. All you’re doing is eating through your batteries and extending the flash recycle time between each shot.

Gary Fong Lightsphere

Gary Fong has made a lot of money off his Lightsphere product and some people swear that it works much better than a basic diffuser cap. Is it? Take a look at it: is it that much different to the standard diffuser cap? It’s a little bit bigger so it may be a touch softer but it’s still basically a light spreader. When pointed up it’s taller so more light will travel direct to the subject than a standard cap but it will depend on the situation whether that is better or worse for your photo.

One thing you need to consider with larger modifiers like this: what happens if the subject is some distance away? Just like with the distant sun, perspective will just shrink the apparent size of the light and turn it back into a hard light like your bare flash. That extra distance is also going to force the flash to work even harder than usual (and it’s already fighting to push through the piece of plastic).

Rogue Flash Bender – a flexible bounce card which can be adjusted into different shapes to help put the light where you want it. A basic retractable bounce card is built into the head of many speedlights.

So… Lightspheres and diffuser caps are great, but only when it makes sense to use them. It’s important to look around at the space you’re shooting in and choose the appropriate tool to get the results you want. If you want a bigger light but don’t want to spray it sideways, maybe use a small softbox attachment that only sends the light forward. Or fire the flash up to the ceiling but use some kind of bounce card attached to the back of the flash so that some of the light also sprays directly forward.

Like so many things in photography, the trick is understanding a little of how your tools work and knowing when they can be useful to you.

If you’ve found the above rant useful, you may like to sign up to one of my regular outdoor lighting workshops that I run in the Melbourne CBD. I run those through my meetup.com group Melbourne Photography Education.

You might also be interested in my companion article on how to blend flash/ambient for event photography.

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